Fill a pot with water and set to boil
While the water is boiling finely chop or grate your onion
Once the water has boiled add in your pasta and cook for approximately 10 minutes or according to package directions
In a second pot on medium heat add in the butter and onions, cooking until the onion begin to soften
Add in flour, stirring constantly to make a roux
Once the roux is cooked ( 1- 2 minutes) add the milk, stirring occasionaly until it thickens
To finish the sauce add in the cheese and season with salt and pepper
Finnally, drain the past once it is cooked and add it to the sauce
Serve and enjoy :)